There has been disclosed an AC voltage detection device having a zener diode, a photocoupler, and a control means for measuring the duration of a signal output from the photocoupler. The photocoupler includes a primary-side light emitting diode and a secondary-side phototransistor. In this technology, in the case where the AC voltage is lower than a predetermined threshold close to 0V, the secondary-side phototransistor is turned on; whereas in the case where AC voltage is higher than the predetermined threshold, the secondary-side phototransistor is turned off.
Therefore, the control means can obtain an output signal corresponding to the ON-OFF state of the secondary-side phototransistor. Further, since the OFF durations of the output signal lengthen as the amplitude of the AC voltage increases, the related-art AC voltage detection device specifies the value of AC voltage on the basis of the OFF durations.
However, in the field of AC voltage detection devices, new methods of detecting an AC voltage by the secondary side unlike the above-described method of the related art are demanded.